1. Field of the Invention
This invention relates to a system for controlling a temperature of a molten resin in a cylinder of an extruder to an optimum temperature.
2. Prior Art
In the extruding of a plastics material or resin, it is necessary to control temperatures of the resin in preselected regions of a cylinder of an extruder to respective optimum temperatures so as to increase an extrusion rate, reduce the scorching of the resin and lower energy costs involved. In addition, this is necessary particularly when an extruded resin is used as an electrical insulation for wires and cables. The reason is that electrical characteristics of the insulation is adversely affected if the resin insulation is not properly temperature-controlled during the extruding operation.
FIG. 1 shows a conventional extruder 1 with a resin temperature control means. A plurality of holes 3 are formed in a cylinder or barrel 2 at a feed region, a compression region and a metering region, each hole 3 terminating short of the inner peripheral surface of the cylinder 2. Temperature detecting elements 4 are inserted in the holes 3, respectively, to detect the temperatures of the regions of the cylinder 2. Band heaters 6 (or embedded heater) and cooling blowers 7 are mounted on the cylinder 2. In operation, the temperature detecting elements 4 detect the temperatures of the regions of the cylinder 2 and feed temperature detecting signals to temperature regulating devices 5, respectively. The band heaters 6 and the cooling blowers 7 are controlled through the respective temperature regulating devices 5 to control the temperatures of the cylinder regions to their desired temperatures. With this resin temperature control method, however, it has been found that there is a considerable difference between the temperature of the cylinder 2 at each region and the temperature of the molten resin in the cylinder. More specifically, as shown in FIG. 2, a temperature difference .DELTA.T.sub.1 is produced between the temperature of the inner peripheral surface of the cylinder 2 and the temperature of the cylinder at the regions C.sub.2, C.sub.3 and C.sub.4 on account of a variation of the ambient temperature of the extruder 1. Also, as shown in FIG. 3, on account of a variation of the number of revolutions of the screw, the temperature difference between the actual resin temperature and the temperature of the cylinder 2 is further increased in an amount of .DELTA.T.sub.2. Therefore, the molten resin in the cylinder 2 is heated to above its optimum temperature, and the energy is wasted. C.sub.2, C.sub.3 and C.sub.4 in FIGS. 2 and 3 indicate the above-mentioned regions at which the temperature detecting elements are disposed, and correspond respectively to C.sub.2, C.sub.3 and C.sub.4 where the temperature regulating devices 5 are disposed. For example, in the extrusion of polyethylene resin, when the set temperature of the temperature regulating device 5 is 120.degree. C. and the temperature of the cylinder 2 detected by the temperature detecting element 4 is 120.degree. C., the actual temperature of the molten resin is 130.degree. to 140.degree. C. The temperature of the molten resin in the cylinder 2 is further affected by the set temperature of the extruder, the shape of the screw and the kind of the resin (the grade of the resin and so on) as well as the number of revolutions of the screw and the ambient temperature of the extruder. Therefore, it has been found almost impossible to accurately detect the actual temperature of the molten resin through the temperature detecting elements 4 received in the holes 3 of the cylinder 2.
In order to overcome the disadvantages of the above conventional resin temperature control method, another conventional resin temperature control system is proposed in Japanese Patent Application Laid-Open No. 55-121042. In this system, first and second temperature detecting elements are mounted in a wall of an extruding cylinder, the first temperature detecting element being disposed adjacent to the inner peripheral surface of the cylinder while the second temperature detecting element is disposed adjacent to the outer peripheral surface of the cylinder. The first and the second temperature detecting elements are disposed in a plane perpendicular to the longitudinal axis of the cylinder. The first temperature detecting element and a first temperature setting device are electrically connected to a first comparator, the first temperature setting device being adapted to set a target temperature of the portion of the cylinder adjacent to the inner peripheral surface thereof. Similarly, the second temperature detecting element and a second temperature setting device are electrically connected to a second comparator, the second temperature setting device being adapted to set a target temperature of the portion of the cylinder adjacent to the outer peripheral surface thereof. An output of the first comparator is connected to an output of the second temperature setting device. An output of the second comparator is electrically connected to a temperature control device such as a heater and a blower mounted around the cylinder. For example, when the temperature of the portion of the cylinder adjacent to the inner peripheral surface thereof is lower than the target temperature set by the first temperature setting device, the first comparator feeds a positive signal to a line connecting the output of the second temperature setting device to an input of the second comparator. Therefore, the output signal from the second temperature setting device is rectified by the output signal from the first comparator and is applied to the second comparator. Then, the temperature control device is operable in response to the output signal from the second comparator to heat the cylinder to thereby raise the temperature of the portion of the cylinder adjacent to the inner peripheral surface thereof. In contrast, when the temperature of the portion of the cylinder adjacent to the inner peripheral surface thereof is higher than the target temperature thereof, the temperature control device is operated to cool the cylinder. Similarly, when the temperature of the portion of the cylinder adjacent to the outer peripheral surface thereof is lower than the target temperature set by the second temperature setting device and rectified by the output signal from the first comparator, the temperature control device is operated through the second comparator to heat the cylinder. In contrast, when the temperature of the portion of the cylinder adjacent to the outer peripheral surface thereof is higher than the rectified target temperature, the temperature control device is operated to cool the cylinder. With this conventional control system, however, it has been impossible to accurately control the actual temperature of the molten resin, since this temperature control is effected merely by controlling the temperature of the cylinder through the measurement of the temperature of the cylinder wall, and the above-mentioned temperature variation is caused by the number of revolutions of the screw and the ambient temperature of the extruder and etc.
In order to overcome the deficiencies of the above two prior art methods, it has been proposed to provide temperature detecting elements on the inner peripheral surface of the cylinder or on the extruding screw so that they are in direct contact with the molten resin in the cylinder. With this method, the temperature of the molten resin can be measured accurately. However, since the temperature detecting elements are always in contact with the molten resin during the operation of the extruder, the sensitive portions of the temperature detecting elements are susceptible to wear. This required a frequent replacement of the temperature detecting elements. Therefore, such extruders could not be continuously operated for a long period of time. In addition, since the temperature detecting elements are disposed in the cylinder, the molten resin tends to reside in the cylinder. Further, in the case where holes are provided through the cylinder wall to receive the temperature detecting elements with the sensitive portions thereof disposed within the cylinder, cracks tend to develop in the cylinder wall adjacent the through holes.
Japanese Patent Application Laid-Open No. 56-191906 discloses a method of controlling the temperature of a resin in an extruder in which method temperature variations of the molten resin, caused by the number of revolutions of an extruding screw, the ambient temperature of an extruder and so on, are corrected. A plurality of temperature detecting elements are inserted respectively in holes formed in a wall of an extruding cylinder at regions spaced along the length thereof, each hole terminating short of the inner peripheral surface of the cylinder. Each temperature detecting element detects the temperature of the portion of the cylinder wall where it is mounted, and produces a temperature detecting signal which is converted into a digital signal. The microprocessor system is programmed to be responsive to this digital signal to calculate an estimated temperature of the portion of the cylinder wall. Parameter detecting means are provided for detecting parameters which change the temperature of the molten resin in the extruding cylinder, such as the number of revolutions of the extruding screw and the ambient temperature of the extruder. The microprocessor is programmed to be responsive to the parameter data from the parameter detecting means to calculate corrected temperature in accordance with a predetermined program stored in an associated memory and then to add this corrected temperature to the temperature of the cylinder measured by the temperature detecting element to provide an estimated temperature of the molten resin. The microprocessor is programmed to produce output information representative of a difference between the estimated temperature and an optimum temperature of the molten resin, and to produce output information representative of a set temperature of a temperature regulating device in accordance with this difference. The temperature regulating device compares this output information with the signal from the temperature detecting element to produce a temperature control signal. Then, a temperature control means such as a heater and blower is responsive to this temperature control signal to control the temperature of the cylinder, thereby controlling the temperature of the molten resin to the optimum temperature. However, this resin temperature control method has also been found not satisfactory in that the temperature of the molten resin can not be controlled to an acceptable level.
It has been found that there exists a temperature profile in the direction of the thickness of the wall of the cylinder 2, as shown in a graph in FIG. 4. In this graph, the abscissa axis represents the radial distance (mm) from the inner peripheral surface of the cylinder 2. In this case, 0 mm indicates the inner peripheral surface of the cylinder 2, and 50 mm indicates the outer peripheral surface of the cylinder 2. The ordinate axis of the graph represents the temperature of the cylinder 2. Curves C.sub.2, C.sub.3 and C.sub.4 represent temperature profiles of the temperature detecting regions C.sub.2, C.sub.3 and C.sub.4 of the extruder 1 in FIG. 1, respectively. It has been found that the temperature of the inner peripheral surface of the cylinder 2 can be estimated by extrapolating each curve into the inner peripheral surface of the cylinder. The present invention is based upon these findings.